The present invention relates to an image forming apparatus such as a copying machine, a printer, or the like, which employs an electro-photographic or electrostatic recording method, or the like.
In an image forming apparatus such as a copying machine, an LBP, a facsimile machine, a microfilm reader/printer, or the like, first, an unfixed toner image in accordance with the image data of an intended image is indirectly (by transfer) or directly formed on the surface of a recording medium (transfer sheet, electro-facsimile sheet. electrostatic sheet, printing paper, and the like), with the use of an optional image forming method, for example, an electrophotographic image forming method, an electrostatic image forming method, or the like, and developer (toner) composed of thermally meltable resin or the like. The unfixed Loner image is thermally fixed to the recording medium by a heating apparatus (image heating apparatus, or image heating fixing apparatus); the unfixed image is turned into a permanent image by a heating apparatus. There are various types of heating apparatus. One of the most widely used types of heating apparatus is the heat roller type, which comprises: a heat roller as a heating member, the temperature of which is kept at a predetermined level; and a pressure roller as a pressing member, which has an elastic layer and is pressed upon the heat roller. It heats a recording medium, as an object to be heated, and the unfixed toner image thereon, while conveying the recording medium by the pressure roller, with the recording medium pinched by the heat roller and pressure roller.
In recent years, a film heating type heating apparatus has been gradually taking over the above described beat roller type heating apparatus. A film heating type heating apparatus comprises: a solidly supported heating member: a heat resistant film (fixing film), which is conveyed while being kept pressed upon the heating member; and a pressing member for keeping a recording medium, as an object to be heated, pressed upon the heating member, with the interposition of the heating film. It thermally fixes the unfixed image, which has been formed on a recording medium, by conducting the heat from the heating member, to the recording medium, through the heating film.
A film heating type heating apparatus can be used in various fields; not only can a film heating type heating apparatus be used as a fixing apparatus, but also as a means for heating a recording medium, which is bearing an image, to improve its surface properties (glossiness or the like), a means for temporarily fixing an unfixed image, or the like means for heating an object to be heated.
In the case of a film heating type heating apparatus, a heating member with a low thermal capacity, and a thin film, which are rapid in temperature increase, can be used. Therefore, a film heating type heating apparatus enjoys some advantages over a heat roller type heating apparatus. For example, it is smaller in electric power consumption, is shorter in wait time (warmup time), and is smaller in the amount by which the internal temperature or the main assembly of an image forming apparatus is increased by a heating apparatus.
When a film heating type heating apparatus of a pressure roller driving type is in operation, the film is slid on the heating member by rotationally driving the pressing member (which hereinafter will be referred to as pressure roller) kept pressed upon the heating member, with the interposition of the film, so that the film, or the combination of the film and an object to be heated, is conveyed through the compression nip between the heating member and pressure roller, remaining pinched by the heating member and pressure roller. Therefore, as the operation of the heating apparatus continues, the temperature of the pressure roller increases. As a result, the pressure roller increases in diameter due to the thermal expansion of its rubber portion.
Normally, the pressure roller is rotationally driven at a constant rotational angle. Thus, as the temperature of the pressure roller increases, the speed at which an object to be heated is conveyed increases, because the increase in the pressure roller temperature increases the amount of the thermal expansion of the pressure roller diameter, as described above, which results in the increase in the circumference of the pressure roller, and the increase in the circumference of the pressure roller results in the increase in the peripheral velocity of the pressure roller. In other words, the velocity at which an object to be heated is conveyed by a heating apparatus Is affected by the temperature of the pressure roller.
On the other hand, in an image forming apparatus which employs a heating apparatus, such as the above described one, as a thermal image heating apparatus, the velocity at which a recording medium is conveyed, on the upstream side or the heating apparatus, for example, in the image transferring portion, or one of the image processing portion, is kept constant. As the leading end of the recording medium reaches the compression nip (fixing nip) of the heating apparatus, the recording medium begins to be conveyed through the compression nip, remaining pinched by the heating member and pressure roller, before the trailing end of the recording medium comes out of the transferring portion. Thus, when the pressure roller temperature is high, and therefore, the recording medium conveyance velocity in the fixing nip of the heating apparatus has become greater than the recording medium conveyance velocity In the transfer portion as described above, the recording medium is pulled by the heating apparatus. As a result, the portion of an image, which is being conveyed through the transfer portion after the arrival of the leading end of the recording medium at the fixing nip, is blurred.
It is possible to set the recording medium conveyance velocity of the heating apparatus at a velocity lower than the recording medium conveyance velocity of the transferring portion, in consideration of the above described phenomenon that the recording medium is pulled by the heating apparatus. However, if such an arrangement is made, a recording medium is slackened between the transferring portion and fixing portion of the fixing apparatus, while the pressure roller temperature is low. This slack destabilizes the direction in which a recording medium separates from the transferring portion after image transfer, and/or the angle at which a recording medium enters the fixing nip portion of a heating apparatus. As a result, some of the toner particles are scattered from the unfixed toner image, and/or offset occurs in the fixing nip of the heating apparatus. Further, when a recording medium is thick, an image is blurred In the transferring portion due to the abrupt straightening of the recording medium caused by its resiliency.
In order to eliminate the problems resulting from the phenomenon that a recording medium is pulled or slackened by a heating apparatus due to the fluctuation in the velocity at which the recording medium is conveyed through the heating apparatus, while remaining pinched in the fixing nip portion, it was necessary to increases the distance between the transferring portion and the fixing nip portion of the heating apparatus so that the length of the time the recording medium is present in both the transferring portion and fixing nip portion of the heating apparatus is reduced.
However, in the case of an image forming apparatus which is enabled to accept a recording medium substantially longer than the standard recording medium, for example, a recording sheet of an A3 size, the above described solution in fact makes the distance between the transferring portion and fixing portion, across which the recording medium must be conveyed, long enough to substantially increase the size of the main assembly of the image forming apparatus.
Thus, as a solution different from the above described one, Japanese Laid-open Patent Application 7-261584 proposes a method in which the amount of the slack of a recording medium, or the conveyance velocity of the recording medium, is detected, and the velocity at which a pressure roller is driven is changed based on the detected amount of the slack, or the detected recording medium conveyance velocity. In the case of this method, however, the rotational velocity of the pressure roller cannot be adjusted until the first recording medium reaches the fixing device. Thus, it is probable that when a recording medium is conveyed at a high velocity, the adjustment of the rotational velocity of the pressure roller will fall behind.
There has also been proposed a method in which the pressure roller temperature is measured, and the velocity at which the pressure roller is driven is adjusted based on the amount of the pressure roller expansion estimated from the detected pressure roller temperature. This method, however, is greater in adjustment error, failing to drive the pressure roller at a proper velocity.
Japanese Laid-open Patent Application 8-190298 proposes another method. According to this method, the aforementioned fixing film is provided with a plurality of reflective plates, and the peripheral velocity of the film is detected by reading the period at which a reflection type sensor is turned on and off. Then, the velocity at which the pressure roller is driven is varied so that the measured peripheral velocity of the film remains constant. This method, however, requires the film to be provided with a plurality of reflective plates, increasing cost. Further, this method requires the intervals of the plurality of reflective plates to be equal. In order to make the intervals equal, the film must be highly accurately processed, which is rather difficult. In addition, this method requires the signals for controlling the peripheral velocity of the film to be continuously processed at a high speed, requiring therefore a more expensive CPU.
The primary object of the present invention is to provide an image forming apparatus capable of optimizing the amount of the slack of a recording medium between the transferring portion and fixing portion of the image forming apparatus.
Another object of the present invention is to provide an image forming apparatus capable of varying the peripheral velocity of the rotational fixing member.
According to an aspect of the present invention, there is provided an image forming apparatus comprising:
an image bearing member for carrying a toner image, the image bearing member being rotatable at a predetermined peripheral speed;
transfer means which forms with the image bearing member a transfer nip for transferring the toner image from the image bearing member onto a transfer material;
fixing means for fixing the image transferred from the image bearing member on the recording material, the fixing means including a fixing rotatable member and a pressing rotatable member which is press-contacted through the fixing rotatable member to form a fixing nip therebetween, the fixing nip being effective to nip and feed the transfer material;
control means for controlling a peripheral speed of the fixing rotatable member, wherein the following is satisfied:
Pmin greater than Lmin
xe2x80x830 less than Xxe2x89xa6xcex4L/Pmax
V0: the peripheral speed of the image bearing member;
Vf: a feeding speed of the fixing means;
Lmin: a minimum path length between the transfer nip and the fixing nip;
Lmax: a maximum guide path length between the transfer nip and the fixing nip;
xcex4L=Lmaxxe2x88x92Lmin; an amount of loop formed between the transfer nip and a fixing nip;
Pmax: a maximum length of the transfer material on which the image forming apparatus is capable of forming the toner image:
Pmin: a minimum length of the transfer material on which the image forming apparatus is capable of forming the toner image;
X: a fluctuation rate of the feeding speed Vf or the fixing means relative to the peripheral speed V0 of the image bearing member during control of the peripheral speed of the control means.